The individually movable needles of a knitting machine are normally housed in tricks and have, on or associated with them, lifting butts engageable with a lifting cam at each selection station whereby the needles may be lifted to knitting position. The needles also have, or are associated with, selection butts engageable with selectors to determine whether the lifting butt engages the lifting cam or is retracted into the trick whereby it misses the lifting cam and the needle does not knit.
With a circular knitting machine having a typical number of needles, such as 1728, and operating at a typical speed, say 30 r.p.m, the needles move past the selectors, or vice-versa in a rotary cambox machine, at a rate approaching 1 per millisecond so that the time available for selection is necessarily very short and decreases as the speed of the machine increases. In order to increase the available time at a given knitting speed, electromagnetically controlled mechanically operated actuators have been stacked in banks of four or six, each operating on only every fourth or sixth selection butt, the selection butts being in echelon or other height-differing formation for this purpose. An alternative has been to have the actuators without moving parts, i.e. selectors operating electromagnetically directly on the selection butts. However, even in this type of selection mechanism timing is very critical since a selection butt held by the selector has to pass out of the field of influence of the selector before the selector can operate to release the next butt following one millisecond behind. It is conventional for the held selection butts to continue to be held on a permanent magnet after selection and if the released selection butts are not released early enough, there is a danger that they may come into the effective field of the permanent magnet and be held rather than released.